1. Field of the Invention
This invention relates generally to apparatus for use in reshaping the cornea of a human eye. More particularly, it relates to human corneal refractive surgery and techniques and apparatus used to model the human eye as an ellipsoid to determine a desired refractive correction.
2. Background of Related Art
First generation ablating excimer laser systems are characterized by relatively large diameter laser beams (6 mm), low laser pulse repetition rates (10 Hz), and mechanical means for shaping the resultant ablation profile. These older generation systems cannot achieve the accuracy required to shape the ablation profiles described in this disclosure, nor do they have the resolution necessary to achieve optimal refractive results. The excimer laser system described herein is a later generation system incorporating a small diameter laser beam (1 mm), operating at relatively high laser pulse repetition rate (100-200 Hz), and incorporating computer controlled x-y scanning to control the ablation pattern.
A model of the human eye is used for the initial and target shapes for corneal refractive surgery. The currently utilized technique for planning refractive surgery assumes that the human eye is a spherical globe of a certain radius of curvature r1, as depicted in FIG. 4A. To correct refractive errors, a spherical globe of lesser or greater curvature is targeted to achieve the ideal patient refraction. The intersection between these two globes provides the amount of material to remove from the patient""s cornea in order to achieve the target refraction.
FIG. 4B depicts a Munnerlyn model wherein a presumed spherical cornea is ablated to a desired spherical shape having a single radius r1xe2x80x2. Using this model, the target corneal surface is always a spherical shape.
However, the cornea is not exactly spherical, and ablation systems and techniques which determined a closest fit sphere to a patient""s cornea where somewhat inaccurate because of the differences between the actual shape of the cornea and the best fit sphere modeling the cornea used by the ablation system.
Corneal ablation surgery was then extended to use initial and target toroidal surfaces instead of spheres to allow for correction of astigmatic refractive error. Astigmatisms are refractive errors that occur along two different radii.
A toroidal surface might be thought of as an xe2x80x98inner tubexe2x80x99 shape, with a major radius r1 (the radius of the whole tube) and a minor radius r2 (the radius of a cross sectional area of the tube), as shown in FIG. 5A. Thus, the art had moved from models having a single radius (i.e., spheres) to models having two radii (i.e., toroids).
The toroidal surface, however, makes the assumption that the eye shape is circular along any meridian through the optical center of the eye""s surface. An exemplary toroidal ablation pattern is shown from above the cornea in FIG. 5B. However, in practice, this is rarely the case, and toroidal surfaces too cause inaccuracies in ablation systems due to the differences between the best fit toroidal surface and the actual shape of the patient"".
The human eye is perhaps best modeled by using an ellipsoid instead of a toroid or a sphere. If an ellipsoid is used to model the human eye, the prolate shapes, or surfaces, of the ellipsoid can be used to more accurately describe the true shape of the eye. A prolate shape is one having its polar axis longer than its equatorial diameter. Substituting the ellipsoid shape instead of the toroid shape can then be used to determine the corneal tissue to remove.
The ellipsoid shape is a surface in which all plane sections perpendicular to any axis of which are ellipses or circles. The normally flatter portions of the ellipsoid shape are considered as oblate or flattened or depressed at the poles. The steeper areas of the ellipsoid are considered as prolate or elongated in the direction of a line joining the poles. The currently applied models of the human eye utilized to plan ablation profiles assume corneal surfaces that approximate an oblate shape. Light rays refracted through the oblate surface are not confocal; i.e. they do not have the same foci.
The result of this inability to achieve a confocal condition frequently results in glare, halos, and loss of night vision.
While the advantages of ellipsoidal modeling of a cornea have been noted, conventional corneal modeling uses a biconic technique which requires the solution of four (4) or more parameters (e.g., two radius values, two eccentricity values). The use of the ellipsoid equation on eccentricity has not been used, only the biconic.
There is thus a need for an ablation laser system and method which utilizes accurate ellipsoidal modeling for precise and realistic refractive correction of corneas.
In accordance with one aspect of the present invention, a prolate corneal modeler in a corneal laser ablation system comprises a corneal measurement input module, and an ellipsoid fitter. The ellipsoid fitter generates a best ellipsoid fit to corneal data input to the ellipsoid fitter using an ellipsoid algorithm having only three degrees of freedom.
A method of modeling a corneal surface in accordance with another aspect of the present invention comprises receiving corneal measurement data, and fitting the corneal measurement data to a best fit ellipsoid having only three degrees of freedom.
A method of ablating a cornea into a prolate shape in accordance with still another aspect of the present invention comprises determining a desired ablation pattern to form a prolate shaped ellipsoid on a surface of the cornea. An eccentricity of the prolate shaped ellipsoid is adjusted to intentionally leave approximately 10% astigmatism on an ablated surface of the cornea to cancel an astigmatic condition on a posterior side of the cornea and any contribution from lenticular astigmatism. The cornea is ablated in accordance with the adjusted prolate shaped ellipsoid.